1. Field of the Invention
The present invention relates to a wireless communication terminal that reduces power consumption when a reception environment is degraded.
2. Description of the Related Art
In data transmission, a data transmission rate (rate of physical layer) that matches between a transmitting-end and a receiving-end is selected, and data is transmitted and received. The transmission rate must be set higher because of the larger the size of a packet to be transmitted per unit of time. Terminals near a base station, such as a wireless terminal in a location within visible range of the base station, are in a favorable mobile radio environment and can transmit data with little frame error/packet error, even when a large transmission rate is assigned thereto.
However, terminals located far from a base station and terminals in a location, such as underground, where reception of radio waves is difficult are susceptible to error factors, such as noise, interference, and attenuation. Therefore, when a large transmission rate is assigned to a terminal, the terminal becomes vulnerable to influence during transmission because the amount of data transmitted at one time is large. Thus, many frame errors/packet errors occur and data transmission may fail. As a countermeasure to a situation as such, in Japanese Laid-Open Patent Publication No. 2003-51781, for example, a technology that performs data transmission by measuring a reception environment of a received signal and determining a transmission rate according to a table that sets transmission rates corresponding to reception environments is disclosed.
Generally, in data communication, one data transmission rate is selected from a plurality of transmission rates (communication speeds) supported between the transmitting and receiving stations and subsequent communication is performed at the selected transmission rate. The technology described in Japanese Laid-Open Patent Publication No. 2003-51781 provides a transmission rate table corresponding to reception environments. Through negotiations in both directions, the reception terminal selects a transmission rate suitable for the mobile radio environment or a transmission rate supported by the transmitting-end thereby enabling communication suited for the terminals.
However, the following issue can be considered when the transmission rate of a transmitting-end is determined unilaterally by the transmitting-end in a one-to-many, single-direction communication, such as broadcast and multicast communication. Namely, when a large transmission rate is assigned to a terminal in a poor mobile radio environment (the value expressing mobile radio environment is small and communication cannot be performed successfully), many frame errors/packet errors occur and data transmission may fail. Because the transmission rate cannot be controlled by the reception terminal, even if the reception terminal changes the transmission rate according to the reception environment, the transmission rate does not match between the transmitting and receiving stations. Thus, communication cannot be performed.
Data communication cannot be established in the situation above even when the transmission rate is changed at the receiving-end and data reception is attempted and thus, this operation becomes a factor in needless power consumption in the terminals. In addition, when data transmission accommodating the transmission rate of the transmitting-end is attempted, if the mobile radio environment is below a certain received signal strength indicator, many frame errors/packet errors occur in a communication at a transmission rate that is not suitable for the mobile radio state and as a result, data communication fails.